Yielding rock bolt

ABSTRACT

A yielding rock bolt having a solid metal shaft with a relatively wide portion and a relatively narrow portion and an anchor member mounted about the shaft. The anchor member has longitudinal bore which is of lesser dimension than the relatively wide portion. The anchor member is mounted about the relatively narrow portion adjacent the wide portion. In use the shaft is extruded through the anchor member to cause the rock to yield as a rock face moves.

FIELD OF THE INVENTION

The present invention relates to a yielding rock bolt.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention there is provideda yielding rock bolt arranged to be inserted into a hole in a rocksurface, characterised by comprising a shaft formed of a solid metalbar, the shaft having a first end and a second end, the shaft having arelatively wide portion adjacent the first end thereof and a relativelynarrow portion adjacent the wide portion, an anchor member having alongitudinal bore mounted about the shaft at the relatively narrowportion and adjacent the wide portion, the longitudinal bore having atleast a portion of lesser dimension than the relatively wide portion

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a yielding rock bolt in accordance withan embodiment of the present invention;

FIG. 2 is a longitudinal cross-sectional view of the rock bolt of FIG.1;

FIG. 3 is a longitudinal cross-sectional view of a shaft having ananchor member mounted thereabout;

FIG. 4 is a view similar to FIG. 3 showing the anchor member and theshaft being formed into a particular profile by swage press members;

FIG. 5 is a view similar to FIG. 3 showing the shaft and the anchormember after being formed by the swage press members shown in FIG. 4;and

FIG. 6 is a longitudinal cross-sectional view of a rock bolt inaccordance with a second embodiment of the present invention formed bythe steps illustrated in FIGS. 3 to 5.

DETAILED DESCRIPTION OF THE DRAWINGS

In the drawings, there is shown a yielding rock bolt 10 comprising ashaft 12 and an anchor member 14 mounted about the shaft 12. The shaft12 is in the form of a solid metal bar. The anchor member 14 has alongitudinal bore 15 as can be seen in FIG. 2, which receives the shaft12. The shaft 12 has a first end 16 and a second end 18. Further, theshaft 12 has a relatively wide portion 20 adjacent the first end 16 anda relatively narrow portion 22 adjacent the wide portion 20 andextending to the second end 18. The anchor member 14 is located adjacentthe wide portion 20 at the narrow portion 22.

Preferably, the longitudinal bore 15 of the anchor member 14 isdimensioned so that the anchor member 14 can fit over the narrow portion22 snugly and engage with an inner end of the wide portion 20 as shownin FIG. 2. However, a portion of the longitudinal bore 15 is of lesserdimension than the wide portion 20.

Further, the shaft 12 is provided with a debonding sheath 24 formed of asuitable material such as plastics material extending along and aboutthe full length of the shaft 12 apart from the region at which theanchor member 14 is disposed.

The rock bolt 10 is also provided with a rock-face engaging plate 26adjacent the second end 18 of the shaft 12. The shaft 12 is formed atthe second end 18 with a screw threaded portion.

The plate 26 is fitted over the screw threaded portion of the shaft 12.Then a washer 28 is placed over the second end 18 and a threaded nut 30is then threadedly engaged with the end 18 to retain the plate 26 andthe washer 28 in place.

Further, it is envisaged that the shaft 12 will be provided with awidened stop portion of increased dimension adjacent the first end 16such as a welded circle 32 formed of relatively hard material inside thesheath 24.

Still further, it is envisaged that a resin mixing paddle 33 may be tackwelded to the end 16 of the shaft 12.

Also, the anchor member 14 has a portion 34 which is relatively wideadjacent the wide portion 20 of the shaft 12. Extending towards thesecond end 18 the anchor member 14 has a portion 36 which tapersinwardly towards the second end 18 of the shaft 12 as can best be seenin FIG. 2.

The anchor member 14 may be manufactured from heat treated steel such as41/40 steel heat treated to harden it. Further, the bore 15 of theanchor member 14 is nitrided to prevent molecular welding between theanchor member 14 and the shaft 12. Thus, it is particularly importantthat the anchor member 14 be treated in the longitudinal bore 15 toprevent welding between the anchor member 14 and the shaft 12.

In use, a hole is drilled into a wall of a rock face and the first end16 of the rock bolt 10 is inserted into the hole until the plate member26 engages with the rock face around the hole. The anchor member 14 isdisposed about the shaft 12 adjacent an inner end of the wide portion 20remote from the first end 16 of the shaft 12 (as can best be seen inFIG. 2).

The drilled hole around the rock bolt 10 is then filled with a bondingmaterial such as resin, grout or expansion shells in known manner. Theanchor member 14 is secured in place by bonding with the bondingmaterial whilst the shaft 12 is capable of sliding longitudinally withinthe hole relative to the anchor member 14 because of the debondingsheath 24.

If movement of rock causes a portion of the rock face to begin to breakaway, this portion of the rock face is held in place by the rock bolt 10being secured at the anchor member 14. However, this movement will causethe wide portion 20 of the shaft 12 to be pulled through the anchormember 14. The rock bolt 10 therefore yields as the rock face movespreventing the possibility of sudden failure of the rock face. In thismovement the wide portion 20 of the shaft 12 being of larger dimensionthan a portion of the longitudinal bore 15 of the anchor member 14 isextruded through the anchor member 14. This provides a predictable andsubstantially constant force on the anchor member 14.

This force continues until the stop 32 engages with the anchor member 14at which point the force applied to the anchor member 14 is increasedconsiderably because the stop portion 32 will not extrude through theanchor member 14. At this stage the shaft 12 is subjected to maximumload which is the ultimate tensile strength of the shaft 12.

It is envisaged that the yielding rock bolt 10 of the present inventioncould be manufactured by a number of techniques. In the embodiment ofthe present invention illustrated in FIGS. 1 and 2 it is envisaged thatthe narrow portion 22 could be formed by taking a solid bar of uniformdimension throughout and then extruding a portion of the bar to form thenarrow portion 22 extending to the second end 18. In this case, thenarrow portion 22 would be relatively long as shown in the accompanyingdrawings.

In a preferred embodiment of the present invention an anchor member 14having a relatively wide bore of substantially uniform dimension isdisposed about a shaft 12 of substantially uniform dimension throughoutits length. The anchor member 14 is then swaged onto the shaft 12 inknown manner so as to reduce the dimension of the longitudinal bore andto reduce correspondingly the dimension of the shaft 12 with which theanchor member 14 is engaged. In this embodiment the shaft 12 would onlyhave a narrow portion 22 adjacent the anchor member 14, which narrowportion 22, is a relatively short section of the shaft 12 adjacent thewide portion.

The preferred embodiment of the present invention is illustrated inFIGS. 3 to 6 of the accompanying drawings.

In FIG. 3 there is shown in a shaft 12 which is of substantially uniformthickness. Mounted about the shaft 12 is an anchor member 14. In thisembodiment, the anchor member 14 is a generally cylindrical member witha longitudinal bore 40. The bore 40 is preferably, as shown, ofsubstantially uniform cross section throughout its length.

The shaft 12 and the anchor member 14 of FIG. 3 are placed in a swagepress which includes a pair of swage press members 41 as seen in FIG. 4.The swage press members 41 are profiled so as to have a relatively deepmid-portion 42. A right hand end portion 43 as seen in FIG. 4 is of lessdepth. The portions 42 and 43 are interconnected adjacent the shaft 12by a sloped portion 44.

At the left hand end of the swage press members 41 as seen in FIG. 4,there is a portion 49 connected to the deep mid-portion 42 by a rightangle portion 45.

In operation, the swage press members 41 are pressed together in knownmanner so as to apply deforming force to the anchor member 14 on theshaft 12 as shown in FIG. 4. This causes the anchor member 14 to deforminwardly in a mid-portion 47 thereof in a shape complementary to theswage press members 41.

The deformation of the anchor member 14 causes a complementarydeformation of the shaft 12 to occur. As can be seen in FIG. 5, the bore40 of the anchor member 14 has an inwardly deformed mid section 47connected to a non-deformed right hand section 48 by a sloped portion50. Further, the anchor member has a left hand portion 52 which isnon-deformed and is connected to the mid-section 47 by a right angleportion 54.

Further, the shaft 12 has an inwardly deformed portion 60 correspondingin shape to the deformed portion 47 of the anchor member 14. Further,the shaft 12 has a sloped portion 62 connecting the deformed portion 60to a non-deformed right hand portion 64.

Further, the shaft 12 has a right angle portion 66 connecting thedeformed portion 60 to a non-deformed left hand portion 68.

As shown in FIG. 6, the yielding rock bolt 10 produced as describedabove in relation to FIGS. 3 to 5, is fitted up in similar manner to theyielding rock bolt of FIGS. 1 and 2. The rock bolt of FIG. 6 is mountedin a hole in a rock face as described above for FIGS. 1 and 2. Onceagain, the rock bolt of FIG. 6 holds the rock face in place if a portionof the rock face begins to break away. The movement of the rock facecauses the wide portion 64 of the shaft 12 to be pulled through theanchor member 14 with the sloped portion 62 leading.

The anchor member 14 causes deformation of the non-deformed portion 64of the shaft 12. Thus, the shaft 12 is extruded through the anchormember 14. This produces a predictable and substantially constant forceon the anchor member 14.

Modifications and variations as would be apparent to a skilled addresseeare deemed to be within the scope of the present invention

1. A yielding rock bolt arranged to be inserted into a hole in the rocksurface, comprising a shaft formed of a solid metal bar, the shafthaving a first end and a second end, the shaft having a relatively wideportion adjacent the first end thereof and a relatively narrow portionadjacent the wide portion, an anchor member having a longitudinal boremounted about the shaft at the relatively narrow portion and adjacentthe wide portion, the longitudinal bore having at least a portion ofless dimension than the relatively wide portion.
 2. A yielding rock boltaccording to claim 1, wherein the narrow portion of the shaft is arelatively short section of the shaft adjacent the wide portion.
 3. Ayielding rock bolt according to claim 1, wherein the narrow portion ofthe shaft extends from the wide portion to the second end of the shaft.4. A yielding rock bolt according to claim 1, wherein a debonding sheathis mounted about the shaft in regions thereof apart from the anchormember.
 5. A yielding rock bolt according to claim 4, wherein thedebonding sheath extends along the full length of the shaft apart fromthe region at which the anchor member is disposed.
 6. A yielding rockbolt according to claim 1, wherein the anchor member is formed of heattreated steel.
 7. A yielding rock bolt according to claim 6, wherein theanchor member has a relatively wide portion adjacent the wide portion ofthe shaft and a portion tapering inwardly towards the second end of theshaft.
 8. A yielding rock bolt according to claim 6, wherein thelongitudinal bore of the anchor member is treated to prevent stickingbetween the anchor member and the shaft.
 9. A yielding rock boltaccording to claim 8, wherein the anchor member is nitrided in thelongitudinal bore to prevent sticking between the anchor member and theshaft.
 10. A yielding rock bolt according to claim 1, wherein a rockengaging plate is mounted about the shaft adjacent the second endthereof.
 11. A yielding rock bolt according to claim 1, wherein a stopportion is mounted about the shaft adjacent the second end thereof. 12.A yielding rock bolt according to claim 11, wherein the stop portion isa welding ring of relatively hard material.
 13. A yielding rock boltaccording to claim 1, wherein a mixing paddle is attached to the firstend of the shaft.
 14. A yielding rock bolt according to claim 2, whereinthe anchor member initially has a substantially uniform longitudinalbore of sufficient dimension to fit over the shaft, and the relativelynarrow portion of the shaft is formed by placing the anchor member in aswage press so as to deform the anchor member to form at least a portionof the longitudinal bore of reduced dimension and a correspondingportion of the shaft of similarly reduced dimension.
 15. A method ofsecuring a rock face by drilling a hole therein, inserting a yieldingrock bolt according to claim 1 into the hole with the first endforemost, filling the hole with bonding material such that if anadjacent portion of the rock face begins to breakaway the wide portionof the shaft is extruded through the anchor member so that the rock boltyields as the rock face moves.